How do I add grunge & rusting texture to a pre-made material? [CYCLE]
You mix nodes with Mix SHADERS use "fac" to blend between both influences. And then connect that last node to the Material output.

Here´s a 4 minute vid on how to make rust: https://youtu.be/hccqcCIGGUw
When you're done, connect it to the second plug on your mix node (the first one is your glass shader).